@Andy: Under Karmic, I just installed xz-utils and ignored the warning, and everything worked fine.
I would be surprised if they were to change this is Karmic, since it's not an LTS, and changing it would impose a huge testing burden to ensure that it worked with everything. But I'm not an Ubuntu developer, I just report bugs, so you may not want to take my word for it.
@Andy: Under Karmic, I just installed xz-utils and ignored the warning, and everything worked fine.
I would be surprised if they were to change this is Karmic, since it's not an LTS, and changing it would impose a huge testing burden to ensure that it worked with everything. But I'm not an Ubuntu developer, I just report bugs, so you may not want to take my word for it.